The invention relates to the field of processes for thin-film deposition and for crystalline growth of materials on a substrate. The invention also relates to a reactor for implementing this process.
For example, it may be a process for the growth of binary compounds. Certain binary compounds do not exist in the liquid state and large crystals of these compounds, allowing homoepitaxial growth, are not available either. This is especially the case for silicon carbide (SiC) and aluminum nitride (AlN).
For SiC in particular, crystals are obtained by the Acheson method and then these serve as seeds for growing them by the Lely method. The crystals thus obtained are of very good crystalline quality but typically their dimensions are of the order of one centimeter. They are too small for industrial exploitation, a growth method capable of growing them up to 5 to 10 cm is therefore necessary. The so-called modified Lely method is currently the only industrial method for producing SiC as 6H or 4H polytypes. It consists in subliming a particulate SiC filler at 2300xc2x0 C. and in condensing it on a seed placed above it at 2100xc2x0 C. It is not without drawbacks, most particularly because of the temperature at which the growth has to be carried out: 2300xc2x0 C. The equipment for raising to these temperatures is very expensive and the difficulties encountered in order to increase the size of the crystals are very considerable. Moreover, the crystals obtained by this method have microchannels deleterious to the production of large power components.
The crystalline growth of SiC crystals by high-temperature chemical vapor deposition (CVD) and growth in liquid phase give high growth rates but do not make it possible to grow, laterally, i.e. mainly in the plane of the deposition, crystals whose dimensions in this plane are satisfactory.
A xe2x80x9clow-temperaturexe2x80x9d CVD method also exists for the growth of SiC, which makes it possible to grow SiC on silicon substrates of very large dimensions, but the quality of the resulting layers is highly insufficient for the fabrication of electronic components because of the presence of a high density of dislocations due to the crystal lattice mismatch between the layer and the substrate.
The situation in the case of AlN is even less favorable since no supplier of crystals of this material exists.
One objective of the present invention is to provide a process and a reactor making it possible to improve the crystalline quality of crystals obtained by growth from a liquid phase on a substrate.
This objective is achieved by virtue of the process according to the invention, which is a process for the crystalline growth of a material, on a solid first material, from a molten material on the solid first material, characterized in that it comprises:
a step (a) of growing the first material (100) on a substrate (10) consisting of a second material (200),
a step (d, dxe2x80x2) in which crystalline tips of the first material (100) are grown from the interface between the first material (100) and the molten material,
a step (f, fxe2x80x2) consisting in growing, laterally, in a plane mainly parallel to that of the free surface of the molten material, crystals from the crystalline tips.
This is because growth via tips allows the generally high dislocation density to be reduced on account of the fact that the first material has itself many dislocations, for example because of a lattice mismatch between the first material and the substrate on which the first material is grown heteroepitaxially, whereas toward the end of the tips, which are in the liquid, on the opposite side from the surface of the first material, stress relaxation occurs, which results in a slight decrease in the number of dislocations, but even for a constant dislocation density, because of the small surface area of each tip, the latter only has a few dislocations.
Advantageously, the process according to the invention comprises a step consisting in reversing the direction of the temperature gradient.
Thus, when these tips have reached about ten micrometers in height, a reversal of the temperature gradient causes lateral growth from the top of these tips. The dislocations, which were very numerous at the surface of the first material, are few in number at the top of the tips and very rare in the crystals which have grown laterally. These crystals are perfectly oriented with respect to one another and coalesce to form a single monocrystal of very high crystalline quality when the thickness becomes sufficiently great. The maximum diameter of the monocrystal is related to the maximum diameter of the starting substrate, for example 300 millimeters in the case of SiC/Si.
The invention also relates to a crystalline growth reactor for implementing the process according to the invention, characterized in that it comprises heating means making it possible to generate a temperature gradient perpendicular to the free surface of the molten material.
This is because the presence of a temperature gradient makes it possible to obtain growth via tips which extends in the direction of the gradient rather than two-dimensional growth parallel to the plane of the first material.
Further benefits, objects and advantages of the invention will appear on reading the detailed description which follows.